The goal of the proposed research is to elucidate the mechanism whereby vasopressin promotes transcellular water movement in responsive epithelial tissues. The specific objective is to determine the role of microtubules and microfilaments in this action of vasopressin, and to examine the possibility that the cytosolic calcium ion concentration plays a role in the action of the hormone. Functional studies on the movement of water, sodium and urea in response to vasopressin will be carried out in isolated toad bladders; the effects of agents which may influence (1) microtubule structure and/or function (2) the cytosolic calcium ion concentration will be examined. Biochemical studies of oxygen consumption and calcium uptake by mitochondria isolated from bladder epithelial cells will be performed. Electron microscopic studies on the state of assembly and distribution of microtubules, and of actin filaments, in the bladder epithelial cells will be carried out, using stereological and cytochemical techniques.